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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="spirit_x3.tutorials.semantic_actions"></a><a class="link" href="semantic_actions.html" title="Parser Semantic Actions">Parser Semantic
      Actions</a>
</h3></div></div></div>
<p>
        The example in the previous section was very simplistic. It only recognized
        data, but did nothing with it. It answered the question: "Did the input
        match?". Now, we want to extract information from what was parsed. For
        example, we would want to store the parsed number after a successful match.
        To do this, you will need <span class="emphasis"><em>semantic actions</em></span>.
      </p>
<p>
        Semantic actions may be attached to any point in the grammar specification.
        These actions are polymorphic function objects that are called whenever a
        part of the parser successfully recognizes a portion of the input. Say you
        have a parser <code class="computeroutput"><span class="identifier">p</span></code>, and a polymorphic
        C++ function object <code class="computeroutput"><span class="identifier">f</span></code>. You
        can make the parser call <code class="computeroutput"><span class="identifier">f</span></code>
        whenever it matches an input by attaching <code class="computeroutput"><span class="identifier">f</span></code>:
      </p>
<pre class="programlisting"><span class="identifier">p</span><span class="special">[</span><span class="identifier">f</span><span class="special">]</span>
</pre>
<p>
        The expression above links <code class="computeroutput"><span class="identifier">f</span></code>
        to the parser, <code class="computeroutput"><span class="identifier">p</span></code>. <code class="computeroutput"><span class="identifier">f</span></code> is expected to be a polymorphic function
        object with the signature:
      </p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Context</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
</pre>
<p>
        We can also use C++14 generic lambdas of the form:
      </p>
<pre class="programlisting"><span class="special">[](</span><span class="keyword">auto</span><span class="special">&amp;</span> <span class="identifier">ctx</span><span class="special">)</span> <span class="special">{</span> <span class="comment">/*...*/</span> <span class="special">}</span>
</pre>
<p>
        From the context, we can extract relevant information:
      </p>
<div class="table">
<a name="spirit_x3.tutorials.semantic_actions.parse_context_access_functions"></a><p class="title"><b>Table 2. Parse Context Access Functions</b></p>
<div class="table-contents"><table class="table" summary="Parse Context Access Functions">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Function
                </p>
              </th>
<th>
                <p>
                  Description
                </p>
              </th>
<th>
                <p>
                  Example
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">_val</span></code>
                </p>
              </td>
<td>
                <p>
                  A reference to the attribute of the innermost rule that directly
                  or indirectly invokes the parser <code class="computeroutput"><span class="identifier">p</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">_val</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span>
                  <span class="special">=</span> <span class="string">"Gotya!"</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">_where</span></code>
                </p>
              </td>
<td>
                <p>
                  Iterator range to the input stream
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">_where</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">).</span><span class="identifier">begin</span><span class="special">()</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">_attr</span></code>
                </p>
              </td>
<td>
                <p>
                  A reference to the attribute of the parser <code class="computeroutput"><span class="identifier">p</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">_val</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span>
                  <span class="special">+=</span> <span class="identifier">_attr</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">_pass</span></code>
                </p>
              </td>
<td>
                <p>
                  A reference to a <code class="computeroutput"><span class="keyword">bool</span></code>
                  flag that can be used to force the <code class="computeroutput"><span class="identifier">p</span></code>
                  to fail
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">_pass</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span>
                  <span class="special">=</span> <span class="keyword">false</span></code>
                </p>
              </td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><h5>
<a name="spirit_x3.tutorials.semantic_actions.h0"></a>
        <span class="phrase"><a name="spirit_x3.tutorials.semantic_actions.examples_of_semantic_actions"></a></span><a class="link" href="semantic_actions.html#spirit_x3.tutorials.semantic_actions.examples_of_semantic_actions">Examples
        of Semantic Actions</a>
      </h5>
<p>
        Given:
      </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">print_action</span>
<span class="special">{</span>
    <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
    <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Context</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">_attr</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
        Take note that with function objects, we need to have an <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> with the Context argument. If we don't
        care about the context, we can use <code class="computeroutput"><span class="identifier">unused_type</span></code>.
        We'll see more of <code class="computeroutput"><span class="identifier">unused_type</span></code>
        elsewhere. <code class="computeroutput"><span class="identifier">unused_type</span></code> is
        a Spirit supplied support class.
      </p>
<p>
        All examples parse inputs of the form:
      </p>
<pre class="programlisting"><span class="string">"{NNN}"</span>
</pre>
<p>
        Where NNN is an integer inside the curly braces (e.g. {44}).
      </p>
<p>
        The first example shows how to attach a function object:
      </p>
<pre class="programlisting"><span class="identifier">parse</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">,</span> <span class="char">'{'</span> <span class="special">&gt;&gt;</span> <span class="identifier">int_</span><span class="special">[</span><span class="identifier">print_action</span><span class="special">()]</span> <span class="special">&gt;&gt;</span> <span class="char">'}'</span><span class="special">);</span>
</pre>
<p>
        What's new? Well <code class="computeroutput"><span class="identifier">int_</span></code> is
        the sibling of <code class="computeroutput"><span class="identifier">double_</span></code>. I'm
        sure you can guess what this parser does.
      </p>
<p>
        The next example shows how use C++14 lambda:
      </p>
<pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">f</span> <span class="special">=</span> <span class="special">[](</span><span class="keyword">auto</span><span class="special">&amp;</span> <span class="identifier">ctx</span><span class="special">){</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">_attr</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">};</span>
<span class="identifier">parse</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">,</span> <span class="char">'{'</span> <span class="special">&gt;&gt;</span> <span class="identifier">int_</span><span class="special">[</span><span class="identifier">f</span><span class="special">]</span> <span class="special">&gt;&gt;</span> <span class="char">'}'</span><span class="special">);</span>
</pre>
<p>
        Attaching semantic actions is the first hurdle one has to tackle when getting
        started with parsing with Spirit. Familiarize yourself with this task.
      </p>
<p>
        The examples above can be found here: <a href="../../../../../example/x3/actions.cpp" target="_top">actions.cpp</a>
      </p>
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      Hartmut Kaiser<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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